Field of the Invention
Difficulties are commonly experienced in mounting together integrated optical components (such as a semiconductor laser with optical fibres or planar waveguides, detectors, fibre amplifiers and the like), with sufficient accuracy.
One of the most demanding situations is the provision of accurate alignment between a laser and waveguides. As an example of this requirement, the case of a semiconductor laser coupling to an AsG waveguide has been considered. For maximum power transfer, the laser and guide should exactly butt together. The coupling efficiency is limited by the mismatch in the mode sizes of the two components. For the structures of current interest, this results in an efficiency of 45% or a loss of 3.5 dB.
In practice, it is not possible to achieve exact butt coupling and it is therefore necessary to determine the optimum separation between the laser and the guide where axial alignment sensitivity is at its lowest. For signalling type applications, a coupling efficiency between the laser and waveguide of 10% or a loss of 10 dB is seen as the minimum acceptable value. Taking this figure as a target specification, the maximum horizontal and vertical misalignment allowable has been calculated as a function of separation between the laser and the waveguide. For the current case, it was found that a separation of 6 .mu.m gave the lowest sensitivity to axial misalignment (1.7 .mu.m or 1.9 .mu.m in the vertical or horizontal directions before loss in excess of the target value was obtained).
These calculations do not take into account any angular deviation between the laser and the waveguide and so should be viewed as a "best case" estimate. As a result, the alignment tolerance sought in a motherboard process was taken as less than 1 .mu.m in both the horizontal and vertical directions. If higher coupling efficiencies than 10% are required, (eg in the case of laser pumping for optical amplifiers), it would be necessary to match the mode sizes of the two components by use of, for example, a lensed laser assembly.
It has been known to join two-integrated circuits or chips by a so called "flip-chip" solder bonding technique. The faces of the two integrated circuits which are to be joined each contain a corresponding pattern of metal pads, and one of the integrated circuits is provided with a solder bump on each metal pad, the two integrated circuits are approximately aligned, the solder is melted, and the solder, by surface tension, moves the two integrated circuits into alignment so that the corresponding metal pads on each integrated circuits are exactly aligned. The alignment is determined by the metal pads themselves. While this provides a fairly accurate way of aligning two components, in particular in a case with flip-chips where the two integrated circuits are to be interconnected electrically, as determined above, greater accuracy of alignment is required in aligning integrated optical components.